Embryonic and fetal mortality reduce reproductive performance of lactating dairy cows. The objectives of this study were to reduce pregnancy loss by administering a deslorelin implant (GnRH agonist) during the late embryonic period, to reduce follicular growth, induce accessory corpora lutea, and increase plasma progesterone concentrations. Lactating dairy cows received an implant containing 2.1 mg of deslorelin (Deslorelin group; n = 89) or no treatment (Control group; n = 92) on Day 27 of pregnancy. Pregnancy, ovarian structures and plasma progesterone concentrations were determined on Days 27 and 45, and pregnancy was re-confirmed on Day 90. On Day 45, mean +/- S.E.M. numbers of class 2 (6-9 mm; 0.72+/-0.19) and class 3 (> or = 10 mm; 0.86 +/- 0.12) follicles for cows in the Deslorelin group were lower (P < 0.01) than the numbers of class 2 (1.90 +/- 0.18) and class 3 (1.92 +/- 0.12) follicles for cows in the Control group. On Day 45, the number of accessory corpora lutea for cows in the Deslorelin group (1.80 +/- 0.07) were greater (P < 0.01) than for cows in the Control group (1.31 +/- 0.07). On Day 45, plasma progesterone concentration was increased (P < 0.01) for cows in the Deslorelin group (8.03 +/- 0.33 ng/mL) compared to cows in the Control group (6.40 +/- 0.31 ng/mL). Pregnancy losses did not differ between Days 27 and 45 and Days 45 and 90 for cows in the Control (15.2 and 11.0%, respectively) and Deslorelin groups (20.2 and 10.5%, respectively). However, in the Deslorelin group, pregnancy loss between Days 45 and 90 was lower (P < 0.05) for cows that formed an accessory CL (0%) compared to cows that did not form an accessory CL (16.1%).